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Figure 6.10 Predictions of global sea-level rise. (Modified from Pirazzoli, 1996)
In addition to C02, other so-called greenhouse gases (methane, ozone, nitrous oxide and chlorofluorocarbons) have shown increasing atmospheric concentrations during the last few decades. Some climate models indicate that the warming produced by the increased greenhouse effect is already taking place and has caused sea-level to rise. There are also predictions that the increase in atmospheric C02 will continue over the next few decades. This increase is expected to raise the mean global surface air temperature by between 1.5 and 4.5°C. This temperature rise is expected to produce a volumetric expansion of the oceans and melting of continental glaciers, causing global sea-level rise. Observational data related to thermal expansion are too limited to make global estimates. Model simulations, however, suggest an annual rise of the order of 0.3 ±0.2 mm due to volumetric expansion alone (steric changes). Due to uncertain mass budget measurements and estimates for the Greenland and Antarctic ice sheets, future predictions of global sea-level variations are difficult to make. The Antarctic ice sheet is usually assumed to be in equilibrium or to have a slightly positive mass balance, while the Greenland ice sheet is probably close to balance (e.g. Pirazzolli, 1996). The majority of the world's alpine glaciers have been retreating over the last century, but their contribution to sea-level rise is limited. It has been roughly estimated that glaciers and ice caps have contributed 0.4 ± 0.3mmyr_1 for the recent sea-level rise (IPCC, 1995).
Estimates of the sea-level rise by the year 2100 vary according to their date of publication (Fig. 6.10). The figure demonstrates a certain uncertainty in the future global sea-level rise. The heavy vertical lines in Fig. 6.10 show when the estimate was made for year 2100, the light vertical lines show the estimates for some year before 2100 but extrapolated to 2100, and the letter B only indicates that the author did not estimate the sea-level range. Since the early 1980s, there has been a decline in the estimated sea-level rise predicted for the next century. According to the Intergovernmental Panel on Climate Change (IPCC, 1995), the 'best estimate' for the year 2100 was a global sea-level rise of 66 cm, with high and low estimates of 110 cm and 30 cm, respectively. In a later estimate, Wigley and Raper (1992) proposed a global sea-level rise of 48 cm (high estimate of 90 cm and low estimate of 15 cm) by the year 2100. The IPCC report of 1995 stated that sea-level is expected to rise as a result of thermal expansion of the world's oceans and melting of alpine glaciers and ice sheets. The 'best estimate' models of climate sensitivity and of ice melt sensitivity to warming, including the effects of future aerosol changes, suggest about a 50 cm increase in global sea-level between 1995 and 2100. This estimate is about 25 per cent lower than the 'best estimate' in the 1995 report due to lower temperature projections and improved climate models. Even if greenhouse gas concentrations were stable by ad 2100, global sea-level is expected to rise at a similar rate for centuries after that time.
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